Increasing integration of modern communication networks imposes on apparatus strong requirements in terms of the ability to support different protocols and data rates. Flexibility is a key factor to more cost efficient networking.
Within this framework, data “agnostic” transceivers have been proposed like those described by the so-called XFP MSA Group, XFP and MSA being acronyms for Small Form Factor Pluggable and Multi Source Agreement, respectively.
Specifically, the goal of the XFP MSA Group is to create a specification for a module, cage hardware, and IC interfaces for a 10 Gbit hot pluggable module converting serial electrical signals to external serial optical or electrical signals. The technology is intended to be flexible enough to support OC192/STM-64, 10 G Fibre Channel, G.709, and 10 G Ethernet, usually with the same module. The module design and the volume of production are expected to enable very low cost 10 G solutions.
The modules in question contain clock and data recovery circuits (CDRs) that are able to work at different data rates around 10 Gbit/s. In fact, different standards imply different data rates, namely 9.953 GHz for SONET/SDH, 10.312 GHz for 10 GbE, 10.5 GHz for Fiber Channel (FC), 10.7 GHz for FEC SONET/SDH and 11.1 GHz for FEC 10 GbE.
The acronyms referred to in the foregoing are well known to those of skill in the art, thus making it unnecessary to provide a detailed explanation herein.
These different standards imply different data rates as well as different specifications for optical and electrical receiver and transmitter characteristics, i.e. extinction ratio, power launched, receiver sensitivity. The extinction ratio (ER) is defined as the ratio of two optical power levels P1 and P2 of a digital signal generated by an optical source, e.g., a laser diode, where P1 is the optical power level generated when the light source is “on”, and P2 is the power level generated when the light source is “off”.
As a consequence, in addition to being capable of detecting and tuning to the proper data rate, a transceiver adapted for multi-standard operation must also be capable of complying with the other varying requirements of the standards involved.
This requirement may be a severe one, especially for e.g. transmitter modules using a direct modulated laser (DML) as the transmitting source. In fact, standards like 10 GbE and FC work at higher bit rates but allow lower ERs, while SONET works at a lower bit rate but requires a higher ER.
The bandwidth of a DML is higher at higher currents, while the ER is lower (when keeping the modulation current constant). Consequently, a DML based transmitter can be adjusted to meet either of the specifications/standards considered in the foregoing by changing the bias current.
At present, adjustment to a certain standard is performed mostly at the factory level: once a certain set up is chosen and implemented, this is fixed and cannot be modified by the user. In order to comply with different standards, the module has to meet a subset of specifications that are compatible with all the supported standards. Of course, this is more difficult to achieve than meeting a single set of specifications and both the product yield and eventually the module price may be adversely affected by the desire to comply with such a requirement.
The need is therefore felt for electro-optical communication systems of the kind considered in the foregoing that may easily adapt themselves to different standards thereby ensuring truly multi-standard operation.